EVA['()UATION AT LOW TEMI'KRATURES. 



69 



in fig. 58 is very convenient. Tlie side tnbe is attached to the suction-pipe of an 

 air-pump and into the neck is thrust a rubber stopper carrying a thermometer and 

 a U-shaped glass tube of small bore, the outer arm (36 inches long) ending in a 

 beaker of mercur}-. Heat may he applied by means of a water-bath. By substitut- 

 ing a funnel for the thermometer the same device nuiy be used to hasten the filtration 

 of thick liquids, hard-pointed filter papers being emplo^-ed. 



SKX.SITIVENES.S TO PLANT ACIDS. 



The tests should be made with naalic, citric, lactic, oxalic, and tartaric acids 

 added to neutral beef-broth, peptone-water, or plant-broths, or to synthetic media 



(see Am. Nat., 1899, p. 208). It is best to titrate with or solutions, to acidify 



MM 10 20 



with or ^ — solutions, and to reckon the acidity in cubic centimeters of normal 



II ^ 



solution (— ) required per liter of medium. If pre- 

 ferred, it may be calculated on 100 cc. portions and 

 expressed in per cents, but there is no advantage in this, 

 and it has the disadvantatfe of introducing fractions. 



SKXSITIVENES.S TO ALKALIE.S (POTASSIUM OR .SODITM 

 IIVDKATE). 



Determine in each case the optimum reaction of the 

 medium for growth. For the majority of bacteria this 

 is said to lie between +10 and + 15 of Fuller's scale.f 

 The best neutral litmus paper should be used freely, but 

 acid and alkaline media should be titrated with plienol- 



N N 

 phthalein and — or — solutions. In some media — 

 ^ 10 20 



c.^i,''., gelatin, juices of various plants — the end-reaction 

 with phenolphthalein and caustic soda is not ver}' sharp. 

 In these cases the titration should be stopped at the first 

 trace of change of color. If one adds alkali until the 

 fluid is decidedly red, then a distinct statement to that 

 ^.■'^^ effect should be made, since otherwise no comparisons of 

 any value can be made. All of the writer's + and — refer- 

 ences to media are based on a reaction stopped at the 

 first distinct trace of pink color. As much again alkali 

 must sometimes be added to obtain a deep-red color. 





^ 



Fig. 60.* 



*FiG 60. Stock bottle of :^ sodium hydrate solution. The small bottle at the right holds con- 

 centrated potash liquor to remove the carbon dioxide from the air which enters the bottle. About 

 one-fourth actual size. 



fThe plus and minus on Fuller's scale denotes, respeciively, acid and alkaline media. The 4- 10, 

 for example, means that exactly 10 cubic centimeters of normal alkali must be added to a liter of 

 the culture medium to render it exactly neutral to phenolphthaleir., and, correspondingly, — 10 means 

 that the fluid is alkaline to phenolphthalein and that 10 cc. of norma! acid would need to be added to 

 bring i liter back to the neutral point. The student should not confuse the litmus neutral point and 

 the phenolphthalein neutral point, as they are about 23° apart, c. g.j+ 10 of Fuller's scale (acid side) 

 is distinctly alkaline to litmus. (Consult '95, Fuller, Bibliog., XVI.) 



